The present invention generally relates to devices and processes used for physicochemical treatment of matters, and more particularly relates to a reactor with acoustic cavitation adapted to use ultrasounds and an acoustic cavitation for the continuous physicochemical treatment of fluid matters, in closed loop or in open loop.
Prior techniques for physicochemical treatment of various liquids and liquid mixtures by acoustic cavitation in a frequency range going from about 100 Hz to a few tens of kilohertz are well known. These processes are carried out in acoustic cavitation reactors that are generally of two types: the reactors for tank, with or without external circulation, and continuous external circulation reactors in open loop or closed loop. The frequencies below 20 kHz are generally said to be xe2x80x9cacousticxe2x80x9d, and those above 20 kHz are said to be xe2x80x9cultrasonicxe2x80x9d. Hereinafter, xe2x80x9cultrasonic reactorxe2x80x9d or xe2x80x9cacoustic cavitation reactorxe2x80x9d will be equally used, whatsoever the operating frequency. Generally, in these ultrasonic reactors, an intense acoustic field is produced by means of electroacoustic sources coupled to the liquid volume to be processed which is located in a suitable enclosure. These sources are generally of piezoelectric type or of a magnetostrictive type.
When the acoustic intensity produced in the liquid exceeds a certain threshold which depends on the nature of the liquid, of the temperature, of the pressure and of the gases in solution, a spontaneous production of cavitation bubbles happens in a few microseconds. But then, the implosion of these bubbles produces a phenomenon of extreme violence called xe2x80x9cacoustic cavitationxe2x80x9d. Within the cavitation bubbles, the temperature can largely exceed 5000 K, and the implosion produces spherical shock waves whose acoustic pressure can be over 1000 atmospheres. These extreme microscopical conditions are the sources of the physicochemical phenomenons which are produced nearby: division of the particles, cleaning and erosion of the surfaces, rupture of the molecules, formation of free radicals, acceleration of the chemical reactions, etc. The chemical or xe2x80x9csonochemicalxe2x80x9d applications of the acoustic cavitation have been the subject of many publications, including synthesis works like, for example: xe2x80x9cPractical Sonochemistryxe2x80x94User""s guide to applications in chemistry and chemical engineeringxe2x80x9d by T. J. Mason, Ellis Horwood, Chichester, R. U., 1991.
In all the prior reactors, generally, the acoustic intensity is the highest at the surface itself of the sources or transducers, or at the internal surface of the reactor coupled to these sources. The acoustic cavitation activity is the most intense on these surfaces and decreases rapidly with the distance. It is the case, for example, in the reactor of U.S. Pat. No. 4,556,467 (Kuhn et al.), or in the one of U.S. Pat. No. 5,484,573 (Berger et al.). This undesirable effect is particularly present in all the reactors using vibrating rods or pistons or xe2x80x9chornsxe2x80x9d of small section (a few cm2, of a diameter largely under the wavelength of sound in the liquid), thrusted into the liquid to be processed: in this case, the volume where the interesting acoustic cavitation happens is reduced to only a few cubic centimeters.
According to the well known principles of acoustic diffraction, the conical transducers of the prior art described in U.S. Pat. No. 4,333,796 (Flynn), can thus only produce the cavitation in the neighbourhood of their narrow surface, and not at the center of the reactor as it is alleged. U.S. Pat. No. 4,556,467 (Kuhn et al.) describes a device used for the continuous treatment of liquids with particles in suspension flowing between parallel vibrating plates. However, in this device, the acoustic cavitation occurs with the maximum intensity directly on the internal metallic surfaces of the plates, which is an important drawback as discussed hereinafter. U.S. Pat. No. 5,384,508 (Vaxelaire) describes an acoustic cavitation reactor made up of a metallic tube having a circular section where a liquid can flow, with piezoelectric transducers positioned at uniform intervals along the tube. These transducers induce the longitudinal resonance of the tube whose length is approximately an integer multiple of a quarter of the wavelength of the sound in the material of the tube. It follows that the acoustic pressure varies periodically along the tube and that it is essentially constant in a transversal section. In this way, the intensity of the acoustic cavitation is the highest only in certain zones spaced by a half-wavelength along the tube. Also in this case, the acoustic cavitation occurs on the internal surface of the tube, which is undesirable. U.S. Pat. No. 4,016,436 (Shoh) describes a device highly similar to the former one, with essentially the same drawbacks.
In all of these prior art techniques, the acoustic cavitation has the effect of gradually destroying the surface of the transducers or of the reactors. Furthermore, the particles resulting from this destruction can combine in an undesirable way with the chemical reactants treated by cavitation.
An object of the present invention is to provide an acoustic cavitation reactor, wherein the major drawbacks of the prior art devices are essentially absent.
A subsidiary object of the present invention is to provide an acoustic cavitation reactor, adapted to perform a continuous treatment of important fluid volumes in circulation at highly variable flow rates compared to the prior art methods and devices.
Another subsidiary object of the present invention is to provide an acoustic cavitation reactor which does not cause a gradual destruction of the walls of the tube in which the liquid to be treated flows.
The acoustic cavitation reactor according to the invention comprises a tube made of flexible material. The tube has an outer wall, an inner wall defining a conduit, and opposite ends respectively provided with inlet and outlet openings communicating with the conduit. Electroacoustic transducers are radially and uniformly distributed around the tube. Each electroacoustic transducer is in a prismatic bar shape having a base of a specific width and a head narrower than the base, which head is pressed on the tube. Each electroacoustic transducer includes an electroacoustic motor mounted at the base such that vibrations generated by the electroacoustic motor are amplified at the head by the shape of the electroacoustic transducer. Films of lubricant extend between the heads of the electroacoustic transducers and the tube, in order to produce an acoustic coupling of the electroacoustic transducers with the tube.
In a first mode of operation, the acoustic cavitation reactor according to the invention allows to produce a cylindrical zone of intense acoustic cavitation in the liquid flowing in the tube, centered on the principal axis of the tube, and whose radius can be smaller than the internal radius of the tube, the radius of this cavitation zone increasing with the intensity of the energization current of the electroacoustic transducers.
In a second mode of operation, the acoustic cavitation reactor according to the invention allows to produce a cylindrical zone of intense turbulence and stirring in the liquid flowing in the tube, centered around the principal axis of the tube, the stirring intensity increasing with the intensity of the energization current of the electroacoustic transducers, with relatively little acoustic cavitation.
Preferably, the tube, the electroacoustic transducers and the films of lubricant are disposed in an enclosure that can be sealed, in which the pressure can be maintained at a determined value with respect to the one in the tube.
Preferably, each electroacoustic transducer comprises a solid bar of uniform prismatic section used for the amplification of the vibrations, and whose longitudinal face at the head is pressed on the tube. The amplifying bar can exhibit a distribution of transverse slots or holes extending in a radial plane with respect to the tube.
Preferably, the head of each electroacoustic transducer exhibits a concave surface substantially fitting the outer wall of the tube, as well as a widening facing the tube.
Preferably, the electroacoustic motor of each electroacoustic transducer comprises a piezoelectric or magnetostrictive resonator secured to the amplifying bar at the base of the electroacoustic transducer. Preferably, the electroacoustic transducers are secured to sides of a polygonal structure formed of two rings positioned at the ends of the tube and extending respectively over and under the electroacoustic transducers.
The enclosure can be surrounded by a peripheral radiator and/or a heat exchanger communicating with a set of conduits and nozzles for lubricant distribution in the enclosure.